The present invention relates to an apparatus for controlling an automatic gear transmission for a motor vehicle.
The logic for controlling a conventional electronically controlled automatic gear transmission is based on a shift schedule determined from vehicle speeds and the amount of a throttle value opening (hereinafter referred to as the amount of accelerator opening) shows, by way of example, a shift scheduled map of a shift-up schedule indicated by the solid lines and a shift-down schedule indicated by the dotted lines.
The motor vehicle can stably run on a flat road or a road with a relatively uniform gradient by controlling selection of gear changes based on the above shift schedule.
When the motor vehicle runs on a road with its a non-uniform gradient uniform, i.e., varying frequently, e.g., when the motor vehicle runs uphill as illustrated in FIG. 7, the gear transmission must be repeatedly shifted up and down. A large-size motor vehicle, in particular, may not be driven uphill with the gears in the D range (automatic gear shifting range).
More specifically, with a conventional shift schedule arranged by an ordinary logic, when a running condition varies (for example, the amount of depression of the accelerator pedal is reduced or the vehicle speed is increased), the gear transmission is shifted up based on the shift map even if the engine output is not sufficient in the gear position to which the transmission is shifted up. Since the engine output is insufficient in the selected gear position, the vehicle speed is lowered and hence the gear transmission is shifted down. Thus, the gear transmission is repeatedly and unnecessarily shifted up and down.
According to the shift schedule based on the shift map shown in FIG. 6, the gear transmission is not shifted down unless the vehicle speed is reduced. This transmission control tends to irritate the driver, to increase the time required to go uphill, and fails to utilize the engine power fully. This results in a reduction in the power performance of the engine.
One solution which has been employed has been to provide a manual gear range separately, and allow the driver to select an optimum gear position. This arrangement is however problematic in that the required gear changing operation is cumbersome and the merits of the automatic gear transmission are lost.